In an image forming process adopting an electrophotographic system, an image is generally formed through a charging step 1 of evenly and uniformly charging the surface of a photosensitive member (also referred to as “image-bearing member”); an exposure step 2 of exposing the charged surface of the photosensitive member to a patterned light (conducting optical writing) to form an electrostatic latent image; a development step 3 of developing the electrostatic latent image on the surface of the photosensitive member with a toner to form a toner image (visible image); a transfer step 4 of transferring the toner image on the surface of the photosensitive member to a transfer material; a fixing step 5 of fixing the toner image transferred to the transfer material by heat or pressure; and a cleaning step 6 of removing the toner remaining on the surface of the photosensitive member after the transfer step. In order to prevent the occurrence of a residual image, a static charge-eliminating step for the surface of the photosensitive member may also be provided between the cleaning step and the exposure step.
The photosensitive member is a member obtained by providing a photoconductive layer (photosensitive layer) on an electrically conductive base. In general, as the photosensitive member, is commonly used a function-separated type photosensitive drum obtained by arranging, as a photosensitive layer, a charge generation layer and a charge transport layer in this order on a cylindrical aluminum base (electrically conductive drum base). As the photosensitive member, various kinds of organic photoconductors such as a single-layer type and a reversed lamination type are known in addition to the function-separated type. As the forms of the photosensitive member, not only a drum form, but also other forms such as an endless belt form are known.
In the development step, is known a system, in which a developing roll is arranged in opposition to the surface of a photosensitive member, and an electrostatic latent image on the surface of the photosensitive member is developed in contact with a toner supplied on the developing roll to form a toner image. More specifically, in a developing device, the toner is supplied on the developing roll by a supply roll, the toner on the developing roll is formed into a thin layer by a layer thickness-regulating member, and the toner on the developing roll is brought into contact with the surface of the photosensitive member, on which the electrostatic latent image has been formed, to develop it, thereby forming the toner image. As the toner, is used a developer containing colored resin particles and an external additive.
As a cleaning method for removing the toner remaining on the surface of the photosensitive member after the toner image on the photosensitive member is transferred on a transfer material, is known a cleaning method by means of a cleaning blade. However, as will be described in detail subsequently, it has been found that a toner containing spherical and small-sized colored resin particles is poor in cleaning ability when the toner is applied to the cleaning method by means of the cleaning blade.
An image forming apparatus of the electrophotographic system, such as an electrophotographic copying machine or laser beam printer is required to form an image having high resolution at a high speed. In particular, with the development of high functionalization and formation of color images in recent years, the standard required for the formation of high-definition full-color images comes to be high. With the worldwide spread of image forming apparatus of the electrophotographic system, the image forming apparatus come to be used not only under a normal-temperature and normal-humidity environment, but also under wide environmental conditions from a high-temperature and high-humidity environment to a low-temperature and low-humidity environment. Therefore, there is a strong demand for development of an image forming method capable of forming high-quality images even under such wide environmental conditions.
In order to meet the above demand, various improvements are principally advanced from both points of image forming apparatus and toners. With respect to colored resin particles making up a toner, it is attempted to form particles having a smaller particle diameter and a sharper particle diameter distribution. In other words, the colored resin particles are desirably small in particle diameter and sharp in particle diameter distribution from the viewpoint of forming a high-definition and high-quality image. On the other hand, from the viewpoint of toner properties such as printing density, resolution, fog and cleaning ability, it is not preferable that fine colored resin particles considerably smaller than a predetermined average particle diameter exist in a great amount.
Toners are roughly divided into a pulverized toner with colored resin particles obtained by a pulverization process and a polymerized toner with colored resin particles obtained by a polymerization process. In the pulverization process, a thermoplastic resin is melted and kneaded together with additive components such as a colorant, a charge control agent and a parting agent, and the resultant kneaded product is pulverized and classified to obtain colored resin particles, thereby providing the pulverized toner. The pulverized toner is in an indefinable form, and its particle diameter distribution is broad. In the pulverized toner, a great amount of fine particles are produced by the pulverization. Since a thermoplastic resin having properties easy to be pulverized is used as a binder resin in the pulverized toner, the amount of fine particles formed by excessive pulverization increases when particles having a smaller particle diameter are formed. In order to make the particle diameter distribution of the pulverized toner sharp, a classification process is required. However, such a process is poor in yield because the amounts of coarse particles and fine particles removed by the classification increase in addition to a complicated process.
On the contrary, according to the polymerization process such as a suspension polymerization process, colored resin particles (also referred to as “colored polymer particles”) having a desired average particle diameter and a sharp particle diameter distribution can be obtained. For example, in the suspension polymerization process, a polymerized toner is obtained as colored polymer particles by a process, in which a polymerizable monomer composition containing a polymerizable monomer, and various kinds of additive components such as a colorant and a charge control agent is dispersed as fine liquid droplets in an aqueous dispersion medium, and then polymerized.
According to the polymerization process, colored polymer particles having a spherical form and a sharp particle diameter distribution can be produced. According to the polymerization process, colored polymer particles (referred to as “core•shell type colored polymer particles”) having a core•shell structure can also be obtained by additionally polymerizing a polymerizable monomer for shell in the presence of the colored polymer particles formed after the polymerization. When the glass transition temperature of a polymer component making up the core particles is made low, while the glass transition temperature of a polymer component making up the shell is made high, a polymerized toner excellent in both storage stability (blocking resistance) and low-temperature fixing ability can be produced.
According to the polymerization process, small-sized colored polymer particles having a volume average particle diameter of at most 10 μm, preferably 4 to 10 μm can be easily produced. Even when the polymerized toner is required to be classified for making its particle diameter distribution sharper, there is no need to remove a great amount of fine particles as compared with pulverized toner. Accordingly, the polymerized toner is permitted forming a high-definition image and also suitable for high-speed printing and formation of full-color images.
As described above, the polymerized toner having a small particle diameter bears an extremely important role in forming a high-resolution and high-definition image. However, various problems arise as the particle diameter of the polymerized toner is made smaller. One of such problems is the fact that such a polymerized toner is poor in cleaning ability.
It has been found that a toner containing spherical and small-sized colored resin particles is difficult to clean off the toner remaining on the surface of a photosensitive member after a transfer step, and an image formed tends to lower its image quality by the remaining toner. As a method for cleaning off the toner remaining on the surface of the photosensitive member, is known a method, in which an elastic cleaning blade made of an elastomer is brought into contact with the surface of the photosensitive member to remove the remaining toner. Since the spherical and small-sized colored resin particles (hereinafter may also be abbreviated as “spherical and small-sized toner”) have high adhesiveness to the surface of the photosensitive member, however, a phenomenon that the toner passes through under the cleaning blade (between the cleaning blade and the photosensitive member) is easy to occur.
When a first color toner remains on the surface of the photosensitive member after a transfer step in the formation of an image using color toners, color mixing with color toners used on and after the second development occurs. Thus, a cleaning step for removing the remaining toner is more important than a case of the formation of a single-color image using a black toner. However, an organic pigment used as a colorant in the color toner is generally high in charging ability compared with carbon black used as a colorant in the black toner, so that its adhesiveness to the surface of the photosensitive member becomes great compared with the black toner.
Methods for improving the ability to clean off the remaining toner include a method, in which a charge level of the toner on the surface of the photosensitive member is lowered. According to such a method, however, the charging ability of the toner is further lowered upon long-term used under a high-temperature and high-humidity environment, so that the toner is easy to incur occurrence of fog and lowering of image density.
proposals have heretofore been made as to an image forming method of a cleanerless system (also referred to as “simultaneous developing and cleaning system”) that cleaning is conducted simultaneously with development without using a cleaning blade, and developers using this image forming method [for example, Japanese Patent Application Laid-Open No. 5-188637 (U.S. Pat. No. 5,328,792)] and Japanese Patent Application Laid-Open No. 8-146652]. In the cleanerless system, a development means for developing an electrostatic latent image on the surface of a photosensitive member to form a toner image is combined with a cleaning means for recovering a toner remaining on the surface of the photosensitive member. When the cleanerless system is adopted for forming a full-color image with color toners, however, color mixing between respective colors becomes easy to occur by the recovery of the remaining toner.
For example, Japanese Patent Application Laid-Open No. 8-146652 discloses a color-image forming apparatus with a plurality of image forming units of the cleanerless system arranged in a row along a conveyer belt. In the color-image forming apparatus of such construction, color mixing by re-transfer is easy to occur. The reason for it is that upon transfer of a toner image formed on and after development with the second color toner, a toner of another color, which has been already transferred on a transfer material, is caused to adhere to the surface of a photosensitive drum by adhesiveness between this toner and the photosensitive drum and repulsion force generated by polarity inversion of the toner by a transfer charger between the toner and the transfer material, and the toner adhered to the surface of the photosensitive drum is recovered within a developing device for another color toner than this toner upon simultaneous developing and cleaning.
In the image forming method meeting the formation of color images, a system that a toner remaining on the surface of a photosensitive member is cleaned off every color is preferred. Therefore, a cleaning system by means of a cleaning blade is reconsidered. In the image forming method including a cleaning step of removing the remaining toner by means of the cleaning blade, for example, an image forming method making use of a toner containing boron or phosphorus in a proportion of 0.1 to 100 ppm has been proposed (for example, Japanese Patent Application Laid-Open No. 2002-311634). There has also been proposed a method making use of a cleaning blade with fine particles caused to adhere in an adhesion quantity of 1 to 10 mg/cm2 per unit area to the surface of at least a portion of the cleaning blade, with which an image-bearing member (photosensitive member) comes into contact (for example, Japanese Patent Application Laid-Open No. 2003-280474).
However, the above methods are not yet sufficient from the viewpoint of reducing the adhesiveness between the spherical and small-sized toner and the surface of the photosensitive member are insufficient in cleaning performance under a low-temperature and low-humidity environment in particular. In addition, the charge level of the toner is lowered when image formation is performed over a long period of time under a high-temperature and high-humidity environment, so that such a toner is easy to cause lowering of image density and fogging.
On the other hand, a cleaning blade made of a polyurethane elastomer has been proposed as a cleaning blade for small-sized toners (for example, Japanese Patent Application Laid-Open Nos. 2001-255801 and 2003-12752). However, the mere use of the cleaning blade made of the polyurethane elastomer is insufficient for cleaning off a spherical and small-sized toner under a low-temperature and low-humidity environment.
There has been proposed a method making use of color developers obtained by containing a flowability improver and spherical fine particles having a weight average particle diameter of 0.2 to 2.5 μm as external additives in colored resin particles upon formation of a color image by means of a latent image-bearing member (photosensitive member) composed of an organic photoconductor containing fluororesin powder (for example, Japanese Patent No. 3114020). When the fluororesin powder is contained in the organic photoconductor, however, the photosensitive member tends to lower the coefficient of friction thereof, so that the developability of a toner used may be lowered in some cases to easily create difficulty in obtaining an image having a high printing density.